As a copyright protection method used in distributing digital data such as image data, audio data, and the like on the Internet, digital watermarking attracts a lot of attention. Digital watermarking is a technique for embedding information so as to be imperceptible to a human being. For example, as a digital watermarking technique for a multi-valued image, various methods that exploit the redundancy of the density values of multi-valued pixels are known.
On the other hand, a binary image such as a document image has small redundancy, and it is difficult to apply the digital watermarking technique to such an image. However, some digital watermarking methods that exploit unique features of document images are known. For example, a method of shifting the baseline of a line (e.g., see Japanese Patent No. 3,136,061), a method of manipulating an inter-word space length (e.g., see U.S. Pat. No. 6,086,706 and Japanese Patent Laid-Open No. 9-186603 (U.S. Pat. No. 5,861,619)), a method of manipulating an inter-character space length (e.g., see “Electronic document data hiding technique using inter-character space”, The 1998 IEEE Asia-Pacific Conf. On Circuits and Systems, 1998, pp. 419-422), a method of rotating a character to change its inclination (e.g., see Yasuhiro Nakamura & Kineo Matsui, “Digital Watermarking onto Japanese Documents by Seal Image”, IPSJ Journal Vol. 38, No. 11, November 1997), and the like are known.
However, since a document image has small redundancy, and the conventional methods proposed so far embed information by changing two variables, i.e., the baseline of a line, inter-word space, or rotation of a character, the changed points stand out (i.e., image quality deteriorates considerably). For this reason, embedding of information in a document image may be detected by a third party.